ABSTRACT Poisoning with organophosphorus (OP) pesticides during gestation is a life-threatening condition for both mothers and fetuses. Treatment relies heavily on the use of high doses of atropine to block muscarinic receptor overactivation by acetylcholine (ACh) build up due to OP-induced block of acetylcholinesterase (AChE). However, despite therapeutic intervention spontaneous miscarriages, infant and/or maternal deaths, and postnatal neurological complications (including seizures and cognitive deficits) can ensue. Although rarely taken into account, the non-selective inhibition of all muscarinic receptor (mAChR) subtypes by atropine may be an important determinant of these poor outcomes. Specifically, inhibition of presynaptic mAChRs (mostly M2), which are part of a negative feedback loop that limits ACh release from cholinergic neurons, can exacerbate the OP-induced cholinergic crisis. The pharmacological profile of R,S-trihexyphenidyl (THP), a drug that has been safely used during pregnancy and is approved for treatment dystonia and Parkinson?s disease, makes it an attractive candidate to treat gestational OP poisoning. In contrast to atropine, THP more selectively inhibits M1 and M3 than M2 mAChRs. In addition, THP inhibits as-of-yet unidentified subtypes of neuronal nicotinic ACh receptors (nAChRs). Overactivation of M1/M3 mAChRs and neuronal nAChRs in the placenta and myometrium, and in the cardiorespiratory and nervous systems can contribute to poor health outcomes following acute OP intoxication during pregnancy. Thus, this project will test the hypothesis that, in part by sparing M2 mAChRs and potentially by blocking nAChRs in addition to M1/M3 mAChRs, THP will be more potent and efficacious than atropine to treat gestational OP poisoning. The focus will be on chlorpyrifos (CPF), a widely used OP pesticide currently included in the U.S. Department of Homeland Security Chemical Threat Risk Assessment list of chemicals that may be deployed to poison large numbers of people in terrorist attacks. A multidisciplinary approach, a translationally relevant animal model (the guinea pig), and a placebo-controlled, randomized, blind design that minimizes experimental bias and maximizes scientific rigor will be used to address three aims. Aims 1 and 2 will establish the effectiveness of THP to save lives, reduce signs of acute toxicity, and prevent the development of neurological complications in mothers and fetuses gestationally exposed to a high dose of CPF. Aim 3 will shed light on the mechanisms that contribute to the toxicity of CPF and the antidotal effectiveness of THP. Successful completion of this project will lay the groundwork for the development of more effective antidotes to treat acute CPF intoxication during pregnancy. Identification of therapeutic interventions that can have a positive impact on the health outcomes of populations acutely intoxicated with OP pesticides lends support to the initiative of the World Health Organization to tackle the issue of acute OP pesticide intoxication, particularly in the developing world. In addition, it fulfills an unmet medical need for the effective treatment of victims of a deliberate attack with these pesticides.